A GEOGRAPHIC-INFORMATION-SYSTEM APPROACH TO ESTIMATING THE ORIGIN OF MIGRATORY RAPTORS IN NORTH AMERICA USING STABLE HYDROGEN ISOTOPE RATIOS IN FEATHERS

Auk, The, Jul 2006 by Lott, Casey A, Smith, Jeff P

ABSTRACT.-

Analysis of stable hydrogen isotope ratios in feathers (δD^sub f^) is a promising method for investigating population connectivity in migratory birds. Stable hydrogen isotope ratios in precipitation (δD^sub p^) vary across North America with respect to latitude, elevation, and seasonal air-mass trajectories. A strong relationship between δD^sub f^ and δD^sub p^ at locations of feather growth has been documented for several bird species. Some studies have used measurements of δD^sub f^ to plot the origins of migrants on maps of long-term weighted-average, growing-season North American δD^sub p^ (hereafter "δD^sub p^ maps") using the observed relationship between δD^sub f^ and δD^sub p^ from a reference sample of known-origin birds. The accuracy of this method depends on the strength of the δD^sub f^ and δD^sub p^ relationship and accuracy of the δD^sub p^ maps. Recently, a high-resolution (1-km^sup 2^) model of North American δD^sub p^ was published (Meehan et al. 2004) that accounts for the effect of elevation on δD^sub p^ where previous models did not. We compared δD^sub f^ measurements from a geographically diverse sample of 264 raptor feathers with δD^sub p^ estimates for feather-sample locations. We documented a strong relationship between raptor δD^sub f^ and δD^sub p^ across North America. However, we also documented substantial regional variation in this relationship. We created a "base map" of North American raptor δD^sub f^ that incorporated the regional variation described by our sample. We plotted δD^sub f^ values from migrant Sharp-shinned Hawks (Accipiter striatus) captured in eastern Nevada directly on this map to demonstrate how it can be used to view the origins of migrants.

Received 2 December 2004, accepted 10 October 2005.

Key words: Accipiter striatus, annual cycle, avian migration, geographic information systems, population connectivity, Sharp-shinned Hawk, stopover.

Un Procedimiento Basado en Sistemas de Información Geográfica para Estimar el Origen de las Aves Rapaces Migratorias en Norte América Usando los Cocientes de Isótopos Estables de Hidrógeno Presentes en las Plumas

RESUMEN. - El análisis de los cocientes de isótopos estables de hidrógeno presentes en las plumas (δD^sub f^) es un método promisorio para investigar la conectividad entre poblaciones en las aves migratorias. Los cocientes de isótopos estables de hidrógeno presentes en la precipitación (δD^sub p^) varían a través de Norte América con respecto a la latitud, la elevación y las trayectorias estacionales de las masas de aire. En varias especies de aves, se ha documentado una fuerte relación entre δD^sub f^ y δD^sub p^ en las localidades donde crecen las plumas. Algunos estudios han empleado medidas de δD^sub f^ para ubicar el origen de los migrantes en mapas basados en promedios ponderados de valores de δD^sub p^ medidos a través de varios años en Norte América ("mapas δD^sup p^"), utilizando la relación observada entre δD^sub f^ y δD^sub p^ en una muestra de referenda de aves de origen conocido. La exactitud de este método depende de qué tan estrecha es la relación entre δD^sub f^ y δD^sub p^, y de la exactitud de los mapas δD^sub p^. Recientemente se publicó un modelo de δD^sub p^ de alta resolución para Norte América (Meehan et al. 2004) que, a diferencia de los modelos previos, tiene en cuenta el efecto de la elevación sobre δD^sub p^. En este estudio comparamos medidas de δD^sub f^ de una muestra geográficamente diversa de 264 plumas de aves rapaces con estimados de δD^sub p^ para los sitios donde las plumas fueron obtenidas. Documentamos una fuerte relación entre δD^sub f^ y δD^sub p^ en las rapaces a través de Norte América. Sin embargo, también documentamos que esta relación varía considerablemente entre regiones. Creamos un mapa base de δD^sub f^ para las rapaces que incopora la variación regional descrita por nuestra muestra. Ilustramos el uso de este mapa para determinar el origen de los migrantes ubicando sobre éste los valores de δD^sub f^ medidos en individuos migratorios de la especie Accipiter striatus.

SEVERAL STUDIES HAVE measured stable hydrogen isotope ratios in feathers (δD^sub f^) of birds at migratory stopover or wintering sites to estimate the areas where these feathers were grown (Chamberlain et al. 1997; Hobson and Wassenaar 1997, 2001; Hobson et al. 2001; Meehan et al. 2001; Wassenaar and Hobson 2001; Kelly et al. 2002; Rubenstein et al. 2002; Smith et al. 2003, 2004; DeLong et al. 2005). This method has been described as a major breakthrough in the ability to establish connectivity among breeding, migrant, and wintering populations (Kelly and Finch 1998, Hobson 1999, Webster et al. 2002, Rubenstein and Hobson 2004). However, precise methods for estimating the origins of migrants and for representing these data spatially are still in the early stages of development.

Estimating migrant origins on the basis of δD^sub f^ relies on two basic patterns in biogeochemistry. First, stable hydrogen isotope ratios in weighted-average growing-season precipitation (δD^sub p^) show strong geographic variation across the ranges of many species of migratory birds (Chamberlain et al. 1997, Hobson and Wassenaar 1997). Second, the local environmental signal of δD^sub p^ is transferred to the feathers of birds through their diet during feather growth (Chamberlain et al. 1997, Hobson and Wassenaar 1997, Hobson et al. 1999a). Thus, measurements of δD^sub f^ from migrants will be indicative of the δD^sub p^ of the area where the feathers were grown (Chamberlain et al. 1997, Hobson and Wassenaar 1997). In the case of hatching-year birds, this will be the natal area, because their feathers are typically grown on or near the nest site.